That's because most owners' manuals read as though there are two basic types of automotive maintenance: normal and severe. This has created a motoring population of under-maintainers, over-maintainers and confused car owners in between.

Under-maintainers treat their cars like appliances. You probably know at least one. They use their cars to get to work and run errands. They put gas in them, but never raise the hood to check anything. Under-maintainers believe they are "normal" drivers because they seldom drive their cars at high speeds on the highway, driving them to and from the train station or supermarket--short trips which, they believe, do not stress the vehicle. They may drive only 4,000 miles per year and consider motorists who roll up 15,000 miles a year abusive. Poor, misguided under-maintainers.

Then there are the over-maintainers, who seem to do nothing but tinker with their cars. You see them washing and waxing every weekend. They religiously change the oil and filter every 3,000 miles, as though it may bring them everlasting salvation so they can wax their cars for eternity in heaven. We confess to being a member of this flock.

But most car owners fall in between and are not sure how often to change their oil. The complexity and confusion compound when several drivers share a vehicle. One driver may use the car for numerous short trips (the "severe" type) while the other generally drives many miles at highway speeds (the "normal" type).

Should we split the difference and change the oil between 3,000 and 7,500 miles? Probably. But there is no formula to figure out the optimum oil-change interval. And what factor should we use if we add a quart of fresh oil?

So we sin on the side of waste or negligence. Fortunately for us, salvation has come in the form of the oil-life monitor.

I was under the impression that the "normal" interval could even be on the conservative side if the vehicle is being driven cross county. Don't some OLMs get to zero at well over 10K miles for this kind of driving? I know some manufacturers say change it at 10K miles max regardless of OLM, but not all.